Such heavy duty tires have relatively flat ground contacting tread regions reinforced by a breaker strip assembly or belt which is dedicated to maintaining a consistent contact between the tire and road surface under all conditions. Such reinforcing breaker strip assemblies normally comprise three or more plies of fabric each consisting of rubber coated metallic cords laid parallel to each other and inclined with respect to the equatorial plane of the tire. Often a narrower fourth ply sometimes termed a "slave" ply is arranged radially outside the three main belt plies. This fourth ply has the function of protecting the main breaker strip assembly both in service and particularly in subsequent retreading operations.
In a conventional arrangement the cords of the radially innermost first ply are inclined at a relatively large angle with respect to the equatorial plane of the tire while the cords of the adjacent second and third plies are inclined at relatively small angles with respect to the tire equatorial plane and in opposite directions to each other. Accordingly crossing angles of the cords of the radially innermost three plies form a triangulated structure which, together with the stiffness of the cords themselves, provides an effective breaker belt of high rigidity in the longitudinal and axial directions of the belt, thus keeping the tire tread flat on the road surface.
Furthermore in belt design, it is desirable to stagger the ply endings in the edge regions of the belt by employing plies of differing widths. This gives a progressive reduction in stiffness and minimized stress concentration at the belt edge. Accordingly, the conventional design for truck tire belts is for the widest ply to be the inner of the two low angles plies and the innermost high angle ply and the other low angle ply to be of similar widths. The belt may optionally include a radially outermost and narrowest fourth ply of low angle cords.
Conventionally, the metallic cords of the belt plies used in heavy duty tires comprise steel wires which are surface coated with brass to promote adhesion to their rubber coating. In the cutting of the belt plies from the fabric sheet, the metallic cords are cut at an acute angle to the cord direction resulting in cut faces which are both sharp and devoid of brass coating. Despite the stress level of the belt edges being minimized the factors of sharp cut ends, poor adhesion and the concentration of stress at the cut face combine to promote the breakdown of adhesion and cutting of the rubber at the belt edge. This so called "belt ply looseness" is a common cause of premature tire failure.
Previous attempts to solve the problem of edging rubber looseness include, for example, the disclosure of document FR-A-2358998 which describes a belt for truck tires comprising a narrow high angle ply sandwiched between an inner wide low angle ply and an outer wide low angle ply. However, the two low angle plies have cords lying in the same direction so that, in combination with the oppositely inclined high angle cords, they do not form a triangulated structure. Accordingly such a non-triangulated belt is not very effective in reinforcing the tire tread region.